Wood resin is composed of fatty acids and resin acids, triglycerides, steryl esters, and sterols. Wood resins, as well as other extractives such as lignins, pectins, and phenols, are the major components of pitch deposits. During mechanical pulping, the encapsulated resin is liberated from ray parenchyma cells and resin canals. Some of the dispersed resin droplets precipitate onto fiber surfaces, impairing fiber to fiber bonding and thereby negatively affecting the physical properties of the pulp. Dispersed resin which precipitates later in the pulping and papermaking processes can affect paper machine runnability and reduce paper quality, resulting in increased manufacturing costs.
Traditionally, pitch deposits in pulping and paper manufacturing processes have been reduced by seasoning wood logs and chips, and by the use of physiochemical control agents. For example, cationic coagulant chemicals have been used to fix the extractives to the fibers so that the pitch particles can be removed with the paper products and therefore reduce and/or eliminate the pitch related deposit on the manufacturing equipment. Traditional methods include alum, talc, bentonite, surfactants, pitch stabilizer, detackifiers, and polymers. However, those traditional methods have their limitations due to the complexities of pulping and paper making process, particularly when used with recycled paper.
In order to overcome the limitations associated with the use of physiochemical treatments, biological treatments, such as the use of enzymes, have been developed and widely used for pitch control at newsprint mills. For example, lipases have been used to degrade triglycerides into glycerol and fatty acids to reduce the pitch deposition problems caused by triglycerides. Such treatments, however, can result in increased concentrations of certain pitch components and by-products, such as fatty acids, which would affect machine runnability and product quality. In fact, one of the major challenges in modern mills with closed-cycle water systems is the removal of lipophilic wood extractives that tend to accumulate in pulping and papermaking systems or circuits because they can no longer be sufficiently purged from the system via the water. The increasing degree of water closure in mills is leading to an increase in pitch concentrations, which increases the chances of pitch deposition and the discharge of more heavily concentrated pitch-laden waste water. This makes it more difficult for pulp and papermaking mills to meet state and federal requirements for reducing effluents to meet minimum effluent discharge levels.
Laccase has been used in some cases to treat pulp, see for example, U.S. Pat. Nos. 5,691,193; 6,242,245; U.S. Pat. No. 6,610,172; and International Publication No. WO 92/09741. Other methods have used enzymes such as lipase and cellulase. U.S. Pat. No. 7,125,471 discloses the use of enzyme compositions containing at least one esterase and at least one lipase for treating sludge added to pulp. U.S. Pat. No. 6,939,437 discloses the use of cationic colloidal alumina microparticles, at least one polymer and optionally a cellulolytic enzyme for the treatment of pulp. U.S. Published Application Nos. 2007/0261806 and 2006/0048908 disclose enzyme formulations for treating pulp (see also U.S. Pat. Nos. 6,770,170; 6,712,933 for additional treatments including enzymes). The treatment pH may be in the range of 3.5 to 12 and the temperature range may be in a range between 35° C. and 90° C. U.S. Pat. No. 5,616,215 discloses the use of lipase and aluminum salt to treat pulp containing mechanical pitch (triglycerides), wherein the treatment pH was in the range of 4-5.5 and the temperature was in the range of 30-70° C.
However, many mechanical pulping mills are operated at physical and chemical conditions where current commercially available, “normal” enzymes will be deactivated and lose their functions or activities. For example, thermomechanical pulping (TMP) mills usually are run at temperatures ranging from 70 to 98° C. This temperature range is too high for even what is today known as thermophilic enzymes. Thus the application of normal lipolytic enzymes disclosed in the prior art is limited or even impossible at such high temperatures. At present, the application of “normal” enzymes would require cooling of pulp stocks by addition of cold water, prior to enzyme treatment. The additional cooling creates many problems, for example, temperature shock to the system, increased fresh water usage, and increased demand of a mill's wastewater treatment capacity. The addition of cooling water can cause energy loss in a mill. Most mills have designed their TMP plants with a limited stock storage time. The use of additional cooling water means that the stock will be stored at much lower consistency. This reduces the stock storage time that is badly needed for most mills, because they have to run the TMP plants under the real-time-price (RTP) schedule to manage the energy cost, particularly in summer times. For example, the RTP schedule demands the paper mills to run the TMP only at low energy cost hours, usually from 10 PM to 8 AM, to produce the full day pulp needed for paper machine production. If the stock consistency is lowered, the mill won't have enough storage space to store pulp stocks. Therefore, in many mills, lowering the TMP operating temperature to male the enzyme work is simply not viable.
Most tree species, particularly southern pines, have higher contents of extractives in the winter (i.e., the papermaker's “pitch season”) than in the summer. Furthermore, the chemical composition of the winter pitch changes too, that is, there is much higher content of long chain triglycerides (TG) and fatty acids in the tree extractives. Long chain TGs and fatty acids have higher melting points and require high temperatures to remain liquid. TG conversion by lipolytic enzymes is therefore favored thermodynamically at a high temperature. Lipolytic enzymes, being mostly surface-active enzymes, won't work effectively since the TGs and pitch remain solid under temperature ranges where the normal enzymes are active. Additionally, solubility and miscibility of fatty acids and their corresponding salts increase with temperature. High temperature operation helps disperse the pitch in the stock and reduces the risk of forming deposition.
It is an object of the present invention to provide a method of treating the fiber stocks with industrial esterases at a temperature 5 to 20 degree Celsius higher than the enzyme's denaturation temperature.
It is an object of the invention to provide a method for decreasing the total amount of extractives in wood pulp that is effective over relatively short treatment times.
It is a further object of the present invention to provide an esterase formulation which is stable and active at temperature range from 70 to 98° C.
It is a further object of the present invention to provide a method for treating pitch containing pulp at a broad pH 3.0-10.5 with the addition of metal salts, cationic polymers and/or their combinations prior to the treatment using normal industrial esterases.
It is still an object of the present invention to provide a method of treating pitch containing pulp at neutral and alkaline pH with an esterase formulation.
It is also an object of the present invention to provide a method of treating pitch containing pulp at acidic pH with an esterase formulation.
It is also an object of the invention to provide a method of decreasing the total amount of extractives in wood pulp with esterase treatment in order to lower refining energy requirements, and better paper machine runnability.